A young woman is found dead in the Göta Canal, molested and murdered. The case is almost instantly cold: nobody can identify her and where and by whom she was killed. Then a stroke of luck: through Interpol her identity is ascertained; she is Roseanna McGraw, an American tourist who was taking a boat trip in southern Sweden. A meticulous investigation determines that she was murdered aboard the boat by a fellow passenger. Evidence is lacking, but after the suspect is observed at length, a sting operation of questionable ethical status (Beck's own opinion) results in the suspect, a sexual deviant, attacking a female police officer and being arrested.

Martin Beck, Lennart Kollberg and Fredrik Melander are introduced. Later in the novel Åke Stenström makes an appearance in one of his trademark pursuits.

Lesser characters who will make appearances in later novels are detective Ahlberg of the Motala police force, and detective Sonja Hansson of the Stockholm police force. The reader is also introduced to Beck's wife, son and daughter.

The novel has been adapted to film twice. In 1967 the film Roseanna became the first film about Martin Beck, with Keve Hjelm as Beck. In 1993 a new film based on the novel and with the same name were released, this time with Gösta Ekman as Beck.